Effects Of Fibrinogen, Fragment D, And Fragment E On The Streptokinase Induced Activation Of Human Plasminogen

It has been observed by several investigators that fibrinogen acts as a streptokinase cofactor, and accelerates the rate of plasminogen conversion to plasmin in the presence of streptokinase. The purpose of this study was to investigate the streptokinase cofactor activity of fibrinogen and identify the portion of the fibrinogen molecule responsible for this activity. When human lys-plasminogen (Lys77-Asn790) was incubated with human fibrinogen (mole ratio 1:1), and catalytic amounts of streptokinase (mole ratio Pg:SK 250:1), a 6-fold increase in the rate of.plasmin production was observed over that observed in the absence of fibrinogen. This rate increase was not inhibited by 10 mM lysine. A 6 to 7-fold rate increase was also observed for the activation of Val₄₄₂-plasminogen, a low molecular weight form of plasminogen with greatly reduced lysine binding ability. These results suggest that fibrinogen interaction with the lysine binding sites of plasminogen is not necessary for the increased rate of activation observed in the presence of fibrinogen. The ability of plasmin-digested fibrinogen to enhance the rate of streptokinase-induced activation of plasminogen was also investigated. Upon digestion of fibrinogen with urokinase-free plasmin, the cofactor activity increased by 50%. Subsequent isolation of fragments D and E revealed that fragment D was fully active, while no activity was found in fragment E. The activity of fragment D was stabilized by Ca²⁺. Further, it was shown that Ca²⁺ stabilized the structure of fragment D to heat denaturation using differential heat capacity scanning calorimetry. In the presence of Ca²⁺, the Tm was shifted by 4°, from 52°C to 56°C, while the AH value remained unchanged.